The invention generally relates to packing materials and their use in chromatography. In particular, the invention relates to packing materials and their use in High Performance Liquid Chromatography (HPLC). More particularly, this invention relates to a process for increasing the stability and durability of organosilanes on the surfaces of silica packing materials that are used in HPLC.
In recent years there has been an increasing emphasis on using chromatography, especially HPLC, for analyzing mixtures by separating their components. HPLC is an efficient tool that is widely used throughout the analytical community. Typically, as shown in FIG. 3, an HPLC separation is performed with an instrument containing solvent reservoir 1, pump 2, injector 3, stainless steel tubing 4, column oven 5, column 6, UV detector 7, data system 8, and backpressure regulator 9. It recently has been found beneficial to heat the separation column to increase the speed of analysis.
Using the solvent reservoirs, pumps, mixing unit, and injection device, a sample of a material to be analyzed is injected in a flow of an appropriate solvent going through the chromatographic column (containing the packing material). The various components of the sample are separated in the column due to adsorption, absorption, size exclusion, ion exchange, or other interactions with the packing material. The separated components are then detected using the detector. Some detectors that are commonly used include ultraviolet absorption, fluorescence, refractive index, conductivity, electrochemical, mass spectrometry and evaporative light scattering. The data obtained is then processed with an integrator or computerized data system.
One widely used packing material in chromatographic columns is based on silica. One common type of silica packing material contains a lipophilic modified surface for use in reversed-phase separations. Common lipophilic agents used in the derivatization process include reactive organosilanes, including chlorodimethyloctadecylsilane. The silica contains silanol groups on its surface and when the derivatization process uses reactive silanes, some of the silanol groups on the surface of the silica do not react with the silanes.
Typically, up to 50% of the silanol groups remain unreacted during the derivatization of the silica. These residual silanol groups interactxe2x80x94usually through ion exchange, hydrogen bonding, and dipole/dipole mechanismsxe2x80x94with the sample material being analyzed, especially with acidic or basic samples. These unreacted silanol groups can create problems during analysis, including problems ranging from increased retention, to excessive tailing and irreversible adsorption of the sample. In addition, they can provide points of degradation of the silica itself through attack from mobile phase components. One role of the lipophilic organosilane derivatization is to shield the silica surface from dissolution by mobile phase components. Gaps in surface coverage provide access for the mobile phase to dissolve the underlying silica or detach the lipophilic silanes and sweep them away.
There have been numerous attempts to overcome the problems caused by the presence of these unreacted silanol groups. Some approaches have been based on modifying the silica itself, e.g., using ultrapure silica, carbonized silica, or coating the silica surface with a polymeric composition. Other approaches have been based on modifying the separation process, e.g., by adding suppressors (such as long chain amines) to the eluent used in the separation process. Yet other approaches have been based on modifying the unreacted silanol groups, e.g., endcapping the residual silanol groups with different types of silanes such as bidentate silanes; polymeric silanes; highly reactive monomeric silanes; silanes containing 1 to 3 organic groups; and silanes containing various leaving groups like halogens, triflates, alkoxy, acyl, oximes, amines or amine salts. See, for example, Dagger et al. Polymer 40(11) pp. 3243-3245 (1999); Chapter 7 of An Introduction to Modern Liquid Chromatography John Wiley and Sons, New York, N.Y. (1979); J. Chromatogr. 352, 199 (1986); J. Chromatogr. 267, 39 (1983); J Chromatogr. 298, 389 (1984); Anal. Chem. 70(20) pp. 4344-4352 (1998); J. Chromatogr. 797(1-2) pp. 111-120 (1998); Advances in Colloid and Interface Science 6, 95 (1976); Angew Cheme. Int. Ed. Engl. 25, 236 (1986); Walter et al. Advances in Silica Technology for Reserved-Phase HPLC Packings HPLC 99; as well as EP Patent Application No. 129,074, JP Patent Application No. (Kokai) 11335462, and U.S. Pat. Nos. 6,136,438, 6,057,468, 5,968,652, 5,948,531, 5,869,724, 5,869,152, 5,861,110, 5,667,674, 5,576,453, 5,439,979, 5,374,755, 5,2670,377, 5,158,758, 4,996,343, 4,895,968, 4,876,595, 4,874,518, 4,837,348, 4,828,695, 4,778,909, 4,746,572, 4,705,725, 4,634,755, 4,619,984, 4,539,399, 4,318,819, 4,590,167, 4,539,399, 3,795,313, and 3,722,181, the entire disclosures of which are incorporated herein by reference.
Unfortunately, none of these approaches has been completely satisfactory. In particular, none of these approaches have produced organosilane modified particles stable enough to allow their use in chromatography under pH extremes or elevated temperature conditions.
The invention provides increased stability of derivitization agents on the surfaces of packing material used in chromatography. In particular, the invention increases the stability of the organosilanes on silicia surfaces used in chromatography, thereby creating a more durable coating of organosilanes. By increasing its stability, the organosilane entity becomes more resistant to de-bonding and the durability of the underlying surface is enhanced against dissolution. Thus, chromatographic separations are able to be performed at higher and lower pH ranges and higher temperatures.
The stability can be increased through attachment of polydentate silanes which may be formed by pre-polymerization of suitable monomers followed by surface bonding, or by first bonding reactive monomers with appropriate functionality to the surface, followed by cross-polymerization into polycarbosilanes that are very stable against hydrolytic cleavage conditions. The stability can also be increased through attachment of polydentate silanes which are either pre-polymerized and then surface bonded or can be bonded first and then cross-polymerized afterward, yielding a polymerized polycarbosilane backbone that is very stable against hydrolytic cleavage conditions.